[2.19.x] G-10142 Make user attributes endpoint extensible

[jrnorth]

@millerw8 @kcwire

Checklist:

• [ ] Documentation Updated
• [ ] Update / Add Threat Dragon models
• [ ] Update / Add Unit Tests
• [ ] Update / Add Integration Tests

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[jrnorth]

build now

[cxddfbot]

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[cxddfbot]

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